Semiconductor bonding pad

ABSTRACT

Improved bonding pads in an integrated circuit are provided, each having a first bonding pad layer comprising a portion of a top metal layer, and a top bonding pad layer comprising a remaining portion of a deposited bonding pad metal fill layer. The thickness of the bonding pad is greater than the thickness of the top metal layer. The composition of the top bonding pad layer may be different from the top metal layer, so that the composition of each may be independently optimized. A method for forming the improved bonding pads includes deposition of a bonding pad metal fill layer to fill openings in a passivation layer over the first bonding pad layers, and removal of the bonding pad metal fill layer over the passivation layer. An alternative method removes the bonding pad metal fill layer only at areas immediately surrounding locations of the bonding pads, leaving top bonding pad layer and a metal radiation shield layer. Intermediate layers may be employed to improve resistivity or other characteristics of the bonding pads.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 09/195,885,filed on Nov. 19, 1998, titled “Improved Semiconductor Bonding Pad ForBetter Reliability”, now U.S. Pat. No. 6,060,378 which is a continuationof U.S. patent application Ser. No. 08/552,755, filed on Nov. 3, 1995,titled “Improved Semiconductor Bonding Pad for Better Reliability” nowabandoned, both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to the manufacture of semiconductordevices. More particularly, the present invention is directed to novelbonding pads for integrated circuit devices for improved reliability,and methods for forming the same.

2. The Relevant Technology

In conventional semiconductor technology, bonding pads are formed in anintegrated circuit as part of a top metal layer. The top metal layer isdeposited and then patterned to form runners and bonding pads.Passivation layers are then deposited over the top metal layer andpatterned, leaving openings in the passivation layers over the bondingpads. Later, during packaging of individual die, each bonding pad has anend of a wire bonded thereto through application of heat, pressure,sonic energy, other forms of energy, or a combination thereof Anopposite end of each wire is bonded to an inner portion of a packagelead.

The reliability of the bonding process is particularly critical sincethe bonding process occurs so late in the production cycle. Die beingpackaged have typically already been tested and sorted. Any problems inthe wire bonding process thus impact only good die, and do so only aftersubstantial investment in the production thereof.

Secure, reliable bonding of the wire to the bonding pad requires thatthe bonding pad be formed of metals compatible with the bonding process.Aluminum and aluminum alloys are typically employed to achieve the mostreliable bonds. Reliable bonding further requires a certain minimumbonding pad thickness, so that sufficient material is present to form asecure bond.

An undesirable effect of the current technology is that the aboverequirements for reliable wire bonding effectively limit the minimumfeature size of the top metal layer. Metal layers having sufficientthickness for reliable bonds are typically too thick to be easilypatterned at higher resolutions. A relatively thick metal layer requiresa relatively thick layer of photoresist to withstand the longer etchrequired to remove the thick metal layer. But a very high resolutionexposure is generally obtained only in conjunction with a shallow depthof focus which is insufficient to provide high resolution exposurethroughout a thick photoresist layer. Also, as the aspect ratio of themetal lines increases, the difficulty of adequately cleaning and fillingbetween the lines, resulting in decreasing reliability with increasingthickness. Further, aluminum and aluminum alloys are more susceptible toelectromigration than some other metals, which prevents the use of verythin metal lines.

SUMMARY AND OBJECTS OF THE INVENTION

An object of the present invention is to provide improved semiconductorbonding pads for increased bond reliability.

Another object of the present invention is to provide improvedsemiconductor bonding pads for increased device reliability due togreater resistance of the finished die to mobile ionic contamination.

Another object of the present invention is to provide highly reliablewire bonding pads and a tight pitch top metal level in a singleintegrated circuit.

Another object of the present invention is to minimize electromigrationin a top metal level and maximize wire bond reliability in a singleintegrated circuit.

Another object of the present invention is to optimize resistivity in atop metal level and optimize wire bond reliability in a singleintegrated circuit.

Another object of the present invention is to decrease the soft errorrate of an integrated circuit.

Another object of the present invention is to improve reliability of themetalization of an integrated circuit by the use of tight pitch singlelevel metalization.

Another object of the present invention is to provide practical andreliable processes for achieving the foregoing objects.

In accordance with a preferred process of the present invention, a topmetal layer of an integrated circuit semiconductor wafer is depositedand patterned, with a portion of the resulting top metal layer forming afirst bonding pad layer. A first passivation layer is then depositedover the entire surface of the wafer, followed by a second passivationlayer. The first and second passivation layers are then patterned,leaving the first bonding pad layers exposed. A bonding pad metal filllayer is then deposited over the entire wafer surface, and then removedover the first and second passivation layers, resulting in top bondingpad layers formed upon the first bonding pad layers.

The resulting inventive bonding pad is thicker than the top metal layer,and extends upward into the openings in the passivation layers,contacting a sidewall of said openings. The top bonding pad layercontacts the second passivation layer, forming therewith over the firstpassivation layer a barrier against contamination of the firstpassivation layer, which is typically less resistant to contaminationthan the second passivation layer. The top bonding pad layer may alsohave a different composition than the top metal layer, allowingindependent optimization of the characteristics of each.

In another preferred process of the present invention, a raised areasurrounding location of the bonding pad is provided in the layersunderlying the top metal layer. The processing then proceeds as above,but when the bonding pad metal fill layer is removed, only thoseportions immediately above the raised area are removed, resulting inboth a top bonding pad layer upon the first bonding pad layer and ametal radiation shield layer over other areas of the wafer surface.

In accordance with the present invention, one or more intermediatelayers may also be employed between the first bonding pad layer and thetop bonding pad layer to improve resistivity or other characteristics ofthe bonding pad.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained may be more fully explained, amore particular description of the invention briefly described abovewill be rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 is a cross section of a portion of a semiconductor wafer uponwhich the improved bonding pads of the present invention are partiallyformed.

FIG. 2 is a plan view of FIG. 1 taken along the section line 2—2.

FIG. 3 is the cross section of FIG. 1 after a bonding pad metal filllayer has been deposited thereon.

FIG. 4 is the cross section of FIG. 3 after removal of the bonding padmetal fill layer over a top passivation layer, showing a finishedimproved bonding pad of the present invention.

FIG. 5 is a cross section of a portion of a semiconductor wafer havingthereon an alternate embodiment of the bonding pads of the presentinvention including therewith a metal shield layer.

FIG. 6 is a cross section of a portion of a semiconductor wafer havingthereon an alternate embodiment of the bonding pads of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the present invention, an improved semiconductor bonding pad isprovided having a first bonding pad layer comprising a portion of a topmetal layer, and a top bonding pad layer comprising a remaining portionof a deposited bonding pad metal fill layer. The thickness of thebonding pad exceeds the thickness of the runners and other features ofthe top metal layer by at least the thickness of the top bonding padlayer. The thickness of the bonding pad, which requires a certainminimum thickness for a reliable bond, and the top metal layer, which ispreferably thinner than the thickness required for a bonding pad, maythus be independently optimized. The composition of the top bonding padlayer may also be different from the top metal layer, so that thecomposition of each may be independently optimized. Intermediate layersmay be included in the bonding pad between the first bonding pad layerand the top bonding pad layer to improve resistivity or othercharacteristics of the bonding pad. A metal shield layer may also beformed from the bonding pad metal fill layer.

FIG. 1 is a cross section of a portion of a semiconductor wafer uponwhich an improved semiconductor bonding pad of the present invention ispartially formed. A top metal layer 14 has been deposited over anunderlying layer 12. The portion of top metal layer 14 shown may beplaced in electrical contact with underlying circuit devices or lowermetal layers by an electrically conductive plug. A first passivationlayer 16 has been deposited over top metal layer 14 and underlying layer12. A second passivation layer 18 has been deposited over firstpassivation layer 16. First and second passivation layers 16, 18 havebeen patterned, resulting in the removal of first and second passivationlayers 16, 18 over a bonding pad area of top metal layer 14. Theresulting opening in first and second passivation layers 16, 18 has asidewall 60.

According to conventional technology, FIG. 1 would represent a finishedbonding pad. In the present invention, in contrast, the portion of metallayer 14 underlying the opening in first and second passivation layers16, 18 represents only a first bonding pad layer 15. In the presentinvention, at least one additional layer is added to form the completedbonding pad, as will be explained below.

FIG. 2 is a plan view of the structure shown in FIG. 1 taken alongsection line 2—2. FIG. 2 shows a top down view of first passivationlayer 16 and top metal layer 14. From FIG. 2, top metal layer 14 is seento include a runner 13 and a first bonding pad layer 15. Opening 19 infirst passivation layer 16 and second passivation layer 18 (shown inFIG. 1) is also shown, in broken outline superimposed over first bondingpad layer 15.

According to one presently preferred process for forming the improvedbonding pads of the present invention, a bonding pad metal fill layer isdeposited directly over the wafer shown in FIG. 1, then the bonding padmetal fill layer is removed over first and second passivation layers 16,18. Application of this process is illustrated in FIGS. 2 and 3.

FIG. 3 shows the cross section of FIG. 1 after deposition of a bondingpad metal fill layer 20. Bonding pad metal fill layer 20 has beendeposited over the entire wafer surface and has filled the opening infirst and second passivation layers 16, 18, contacting metal layer 14 atfirst bonding pad layer 15. Bonding pad metal fill layer 20 may bedeposited by any conventional process, including but not limited tosputtering, CVD, evaporation and the like.

FIG. 4 shows the cross section of FIG. 3 after removal of the portionsof bonding pad metal fill layer 20 overlying second passivation layer18. The remaining portion of bonding pad metal fill layer 20 forms a topbonding pad layer 21 having a side wall which contacts side wall 60 ofthe opening in first and second passivation layers 16, 18.

Removal of the portions of bonding pad metal fill layer 20 overlyingfirst and second passivation layers 16, 18 may be accomplished by anyconventional process, but chemical mechanical polishing (CMP) ispresently preferred. Second passivation layer 18 is typically andpreferably formed of silicon nitride, which with known techniquesprovides a good stop for CMP. The CMP may be performed slightly beyondthe point of contact with second passivation layer 18 to ensure that allunwanted metal is removed. Such over polishing also results in theslight dishing of the upper surface of the remaining portion of bondingpad metal fill layer 20, as shown in FIG. 4.

The improved bonding pad of the present invention shown in FIG. 4provides many advantages over conventional bonding pads. One advantageis that the bonding pad shown in FIG. 4 may be significantly thickerthan top metal layer 14 alone. The bonding pad of FIG. 4 includes bothfirst bonding pad layer 15, which has the thickness of top metal layer14, and top bonding pad layer 21 which may be up to several times thethickness of top metal layer 14. The thickness of top metal layer 14 ispreferably within the range of about 2,000 Angstroms to about 12,000Angstroms, and most preferably from about 3,000 Angstroms to about 6,000Angstroms, while the thickness of top bonding pad layer 21 is preferablywithin the range of about 5,000 Angstroms to about 20,000 Angstroms.Thus the present invention provides for a thick bonding pad for improvedbond reliability, while simultaneously allowing a thin top metal layerwhich results in tighter pitch capability and improved top metal layerreliability.

Another advantage of the improved bonding pad of the present inventionshown in FIG. 4 is that top metal layer 14 and bonding pad metal filllayer 20 need not have identical compositions. Thus the composition oftop metal layer 14 may be optimized for best resistance toelectromigration and least resistance contacts to underlying conductors,while bonding pad metal fill layer may be optimized for best bondreliability. For example, tungsten or a tungsten alloy could be used asthe top metal layer, with its greater resistance to electromigrationallowing for a tighter pitch than possible with aluminum and aluminumalloys, while aluminum or an aluminum alloy could be used for bondingpad metal fill layer 20 to ensure a reliable bond.

The improved bonding pad of the present invention finds particularlyadvantageous application in a single level metalization system. Theabove noted advantages allow a very tight pitch top level metal, suchthat even dense integrated circuits typically requiring multiple levelmetalization may be completely metalized in a single level. Reducingtypical multiple layer metalization schemes to a single level increasesyield and decreases production times and wafer inventories. Eachreference to a top metal layer or level in the specification or claimsthus should be understood to include within the scope thereof a singlemetal layer or level, which of necessity is also then the top layer orlevel.

Yet another advantage of the improved bonding pads of the presentinvention is provided by the contact of top bonding pad layer 21 withsecond passivation layer 18 as seen in FIG. 4. Second passivation layer18 typically and preferably comprises silicon nitride, with a thicknesspreferably within the range of about 2,000 Angstroms to about 10,000Angstroms. Silicon nitride is hard, durable, and relatively imperviousto many contaminants and chemical agents. Silicon nitride films tend toexhibit high stress, however. An important purpose of first passivationlayer 16 is thus to isolate the underlying structures from the stress ofthe silicon nitride film of second passivation layer 18. An undopedoxide of silicon is typically employed for this purpose, with athickness preferably within the range of about 4,000 Angstroms to about15,000 Angstroms. But undoped silicon oxides readily conduct ioniccontamination, and even doped silicon oxides allow contamination topenetrate more easily than silicon nitride. Thus the present inventionprovides a significant advantage by sealing off first passivation layer16 from any external exposure at the bonding pad site, as shown in FIG.4. This contrasts with FIG. 1, which corresponds to conventional bondingpads, in which first passivation layer 16 is exposed at a portion ofsidewall 60.

Another presently preferred process for producing the improved bondingpads of the present invention is essentially identical to the processoutlined above, except that raised areas surrounding locations of thebonding pads are provided in underlying layers. The required topographycan be provided, for example, in a field oxide layer or in a polysiliconlayer somewhere directly below the desired location. The process thenfollows as before, with deposition and patterning of a top metal layer,deposition of a first passivation layer, deposition of a secondpassivation layer, removal of the first and second passivation layersover bonding pad locations, and deposition and removal by CMP of abonding pad metal fill layer. The bonding pad metal fill layer, however,is completely removed only over the raised areas, and not over theentire second passivation layer.

The result of this process is shown in FIG. 5. Raised areas 40 areprovided in underlying layer 12. As a result, the CMP removes bondingpad metal fill layer 20 over raised areas 40, but stops on secondpassivation layer 18 over raised areas 40, resulting in top bonding padlayer 21 and metal shield layer 22. Metal shield layer 22 helps shieldthe integrated circuit device below it from radiation, improving thesoft error rate of the device. This advantage is provided by thestructure shown in FIG. 5 in addition to those advantages discussedabove with respect to the structure shown in FIG. 4.

The first bonding pad layers and top bonding pad layers of the bondingpads of the present invention need not consist of different materials.The bonding pads of the present invention may also be comprised of morethan two layers. For example, one or more intermediate layers may bedeposited between the first bonding pad layer and the top bonding padlayer, as illustrated in FIG. 6.

In FIG. 6, a thin alloying layer 30 has been deposited over a firstbonding pad layer 15. Top bonding pad layer 21 was then formed onalloying layer 30 from bonding pad metal fill layer 20. Where firstbonding pad layer 15 comprises tungsten and top bonding pad layer 21comprises aluminum, titanium or titanium nitride may for example be usedas thin alloying layer 30 to improve the electrical contact andresistivity between first bonding pad layer 15 and top bonding pad layer21.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A method for forming a bonding pad, said methodcomprising: providing a substrate and a raised structure projectingtherefrom; forming a top layer upon the substrate and the raisedstructure; forming a first passivation layer on the top layer; forming asecond passivation layer on the first passivation layer; forming arecess within the first passivation layer and the second passivationlayer and terminating at the top layer; forming a fill layer within therecess and on the second passivation layer; and forming a first portionof the fill layer that is separated by the second passivation layer froma second portion of the fill layer.
 2. The method as defined in claim 1,wherein forming a fill layer within the recess comprises forming thefill layer within the recess in contact with the top layer.
 3. Themethod as defined in claim 1, wherein forming a recess further comprisesforming sidewall for said recess that is defined by the firstpassivation layer and the second passivation layer.
 4. The method asdefined in claim 3, wherein: the sidewall for said recess has a planarinterface with the fill layer; and the sidewall for said recess has alinear interface with the top layer.
 5. The method as defined in claim3, wherein the sidewall for said recess is perpendicular to the toplayer.
 6. The method as defined in claim 1, wherein providing asubstrate and a raised structure projecting therefrom comprises formingat least one of a field oxide layer and a polysilicon layer.
 7. Themethod as defined in claim 1, wherein: the first portion of the filllayer is within the recess; and the second portion of the fill layer isadjacent to an exposed portion of the second passivation layer that isover the raised structure.
 8. The method as defined in claim 1, whereinforming a first portion of the fill layer that is separated by thesecond passivation layer from a second portion of the fill layercomprise planarizing the fill layer such that the fill layer covers thesecond passivation except over the raised structure where the secondpassivation over the raised structure is exposed.
 9. The method asdefined in claim 1, wherein the first passivation layer is composed ofan oxide of silicon and the second passivation layer is composed ofsilicon nitride.
 10. The method as defined in claim 1, wherein: forminga top layer upon the substrate and the raised structure furthercomprises patterning said top layer to form an elongated runnerextending to a first bonding pad layer; forming a recess within thefirst passivation layer and the second passivation layer and terminatingat the top layer further comprises forming said recess to expose saidfirst bonding pad layer; and forming a fill layer within the recess andon the second passivation layer further comprises forming said filllayer upon the exposed first bond pad metal layer.
 11. The method asdefined in claim 1, wherein: the thickness of said top layer is in therange of about 2,000 Angstroms to about 12,000 Angstroms; and thethickness of the fill layer is in the range of about 5,000 Angstroms toabout 20,000 Angstroms.
 12. The method as defined in claim 1, wherein:the first passivation layer and the second passivation layer arecomposed of different electrically insulative materials; and the toplayer and the fill layer are composed of different electricallyconductive materials.
 13. The method as defined in claim 1, wherein:said top layer comprises tungsten; and said fill layer comprisesaluminum.
 14. The method as defined in claim 1, wherein: forming a toplayer upon the substrate and the raised structure further comprisesforming an alloying layer over said top layer; and forming a fill layerwithin the recess and on the second passivation layer further comprisesforming said fill layer upon said alloying layer.
 15. The method asdefined in claim 14, wherein said alloying layer comprises titanium. 16.The method as defined in claim 14, wherein alloying layer comprisestitanium nitride.
 17. A method for forming a bonding pad, said methodcomprising: providing a substrate and a raised structure projectingtherefrom; forming a top layer upon the substrate and the raisedstructure; forming a first electrically insulative layer on the toplayer; forming a second electrically insulative layer on the firstelectrically insulative layer; forming a recess within the firstelectrically insulative layer and the second electrically insulativelayer and terminating at the top layer; forming a fill layer within therecess and on the second passivation layer; and planarizing the filllayer such that the fill layer covers the second passivation except overthe raised structure where the second passivation layer is exposed. 18.The method as defined in claim 17, wherein planarizing the fill layercomprises a chemical mechanical polishing process.
 19. A method forforming a bonding pad, said method comprising: providing a substrate anda raised structure projecting therefrom; forming a top layer upon thesubstrate and the raised structure; forming a first electricallyinsulative layer on the top layer; forming a second electricallyinsulative layer; forming a recess within the first electricallyinsulative layer and the second electrically insulative layer andterminating at the top layer; forming a fill layer within the recess andon the second passivation layer; and removing a portion of the filllayer that is over the raised structure so as to expose a portion of thesecond electrically insulative layer that is over the raised structure.20. The method as defined in claim 19, wherein removing a portion of thefill layer comprises planarizing the fill layer.
 21. The method asdefined in claim 20, wherein planarizing the fill layer comprises achemical mechanical polishing process.
 22. A method for forming abonding pad, said method comprising: providing a substrate and a raisedstructure projecting therefrom; forming a top layer upon the substrateand the raised structure, forming a first passivation layer on the toplayer; forming a second passivation layer on the first passivationlayer; forming a recess within the first passivation layer and thesecond passivation layer and terminating at the top layer; forming afill layer within the recess and on the second passivation layer;planarizing the fill layer to: expose a portion of the secondpassivation layer that is over the raised structure, leave the filllayer covering the second passivation layer except upon the exposedportion of the second passivation layer that is over the raisedstructure; and form a first portion of the fill layer that is separatedby the exposed portion of the second passivation layer that is over theraised structure from a second portion of the fill layer.
 23. The methodas defined in claim 22, wherein: the first passivation layer and thesecond passivation layer are composed of different electricallyinsulative materials; and the top layer and the fill layer are composedof different electrically conductive materials.
 24. A method for forminga bonding pad, said method comprising: providing a substrate and araised structure projecting therefrom, said raised structure beingselected from the group consisting of a field oxide layer and apolysilicon layer; forming a top layer composed of a refractory metalupon the substrate and the raised structure, wherein said top layerforms an elongated runner extending to a first bonding pad layer;forming a first passivation layer composed of an oxide of silicon andhaving a thickness in the range of about 2,000 Angstroms to about 12,000Angstroms on the top layer; forming a second passivation layer composedof silicon nitride and having a thickness in the range of about 5,000Angstroms to about 20,000 Angstroms on the first passivation layer;forming a recess within the first passivation layer and the secondpassivation layer to expose the first bonding pad layer; forming a filllayer composed of a metal within the recess in contact with the firstbonding pad layer and on the second passivation layer; and planarizingthe fill layer so as to form a first portion of the fill layer that isseparated by an exposed portion of the second passivation layer that isover the raised structure from a second portion of the fill layer. 25.The method as defined in claim 24, wherein: said top layer comprisestungsten; and said fill layer comprises aluminum.
 26. The method asdefined in claim 24, wherein: forming a top layer further comprisesforming an alloying layer over said first bonding pad layer; and forminga fill layer further comprises forming said fill layer upon saidalloying layer.
 27. The method as defined in claim 26, wherein saidalloying layer comprises a titanium.
 28. The method as defined in claim26, wherein said alloying layer comprises titanium nitride.
 29. A methodfor forming a bonding pad, said method comprising: providing a raisedstructure rising above a planar surface of an underlying layer, theraised structure being composed of a material selected from the groupconsisting of silicon dioxide and polysilicon; forming a top layercomposed of a refractory metal upon the substrate and the raisedstructure; forming an electrically insulative first passivation layer onthe top layer; forming an electrically insulative second passivationlayer on the first passivation layer; forming a recess within the firstpassivation layer and the second passivation layer and terminating atthe top layer; forming an electrically conductive fill layer within therecess and on the second passivation layer; and forming a first portionof the fill layer that is separated by the second passivation layer froma second portion of the fill layer.
 30. The method as defined in claim29, further comprising, after forming the recess and before forming thefill layer, forming a layer of a refractory metal or nitride thereofupon the top layer within the recess, wherein said fill layer is formedwithin the recess upon said layer of said refractory metal or nitridethereof.
 31. The method as defined in claim 29, wherein the firstpassivation layer is composed of an oxide of silicon and the secondpassivation layer is composed of silicon nitride.
 32. The method asdefined in claim 30, wherein: the top layer is composed of tungsten;said layer of said refractory metal or nitride thereof is composed oftitanium or a nitride thereof; the first passivation layer is composedof an oxide of silicon; the second passivation layer is composed ofsilicon nitride, and the fill layer is composed of aluminum or an alloythereof.